LESTERVILLE, Mo. — The stone retaining wall around a huge mountaintop reservoir in the Ozarks collapsed before daybreak Wednesday, releasing a billion-gallon torrent of water that swept away at least two homes and several vehicles and critically injured three children, authorities said.

The V-shaped, 600-foot breach opened up just after 5 a.m. at a hydroelectric plant run by St. Louis-based utility AmerenUE, and in a matter of minutes the 50-acre reservoir had emptied with terrifying effect, turning the surrounding area into a landscape of flattened trees and clay-covered grass. “We’ll never see anything like it in our lifetime again,” paramedic Chris Hoover said.

AmerenUE chairman and chief executive Gary Rainwater said that the plant’s automated instruments had pumped too much water into the reservoir and caused it to rupture. A backup set of instruments should have recognized the problem but didn’t, and the utility is trying to figure out why, AmerenUE said.

The three children — ages 7 months, 3 and 5 — were listed in critical condition at a hospital in St. Louis. The two older children had breathing problems; the baby suffered from hypothermia.

authorities said.

The names of the children were not immediately released, but Gov. Matt Blunt said Jerry Toops, superintendent of a state park near the reservoir, and his three children were injured when the water hit their home.

The reservoir, built in 1963, was dug out of the top of 1,590-foot Profit Mountain, with huge, sloping, 90-foot-high walls made of the stone removed from the peak. The reservoir — the upper of two reservoirs at the hydroelectric plant — was lined with concrete and asphalt. A plastic liner was added two years ago because of minor leaks, Rainwater said.

Normally, water released from the reservoir rushes down a 7,000-foot shaft and tunnel and spins the turbines to generate electricity. In Wednesday’s accident, water gushed through the breach and streamed down the side of the mountain and into a valley, draining the reservoir like a bathtub.

At 5:12 a.m., the water level in the reservoir was high, according to AmerenUE. By 5:24 a.m., it registered as low. The water eventually flowed back into the Black River.

Soon after the break, police and the National Weather Service urged the 150 residents of Lesterville, about 150 miles downstream in the sparsely populated area, to move to higher ground. But by midday, once the water had flowed back into the river, the danger had passed.

Emergency workers said they saw two tractor-trailers pushed about 150 yards off the road, two pickup trucks and a car tossed into a field, and a house that Hoover described as “just totally gone.”

The cause of the accident was under investigation by federal regulators and other authorities.

J. Mark Robinson, director of the office of energy projects at the Federal Energy Regulatory Commission, said the plant, including the reservoir, was inspected most recently in August and found to be properly operated and maintained.

AmerenUE serves 2.3 million customers in Missouri and Illinois, and the plant provides about 2 percent of its total electric generation. The floodwaters knocked over some power poles, causing scattered outages, but there was no widespread interruption of power because of the breach.

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